Certain persistent storage devices are used on computer systems in order to provide persistent memory for the functioning of the computer system. For example, a magnetic hard disk drive assembly is well known in the art. However, such persistent memory has some drawbacks. It is time consuming to prepare the magnetic hard drive for reading or writing data. In order to do so, in most cases, it is necessary to physically initiate spinning of the disk, and to position a read/write head over the proper location on the disk.
However, the computer system's microprocessor executes instructions only from fast randomly addressable memory such as DRAM or some other type of volatile electronic memory. In order to initialize this addressable memory to boot the operating system, a small amount of non-volatile boot-up memory is typically provided in an EPROM (electronically programmable read-only memory) or similar device. When the computer is booted, a boot program stored in the non-volatile memory is used. The boot program contains code for reading an initial portion of the operating system on the hard disk that is responsible for loading and initializing the remaining portions of the operating system.
The transfer of the operating system, executables, and data from the hard drive is slow, particularly when the operating system is a large operating system, a file is large, etc. This is prevalent not only in booting the computer system, but also in restoring the computer system from hibernate mode. A significant factor in the transfer time is due to the disk drive spin up speed. A relatively small disk spinning at a relatively slow RPM requires 5 to 6 seconds to spin up and be usable. Larger disks such as multi-platter devices and those spinning at faster RPMs require 10 to 12 seconds or more to spin up. On server systems having hundreds or thousands of disk drives that spin up in a largely sequential manner, the disk spin up time can be over ninety percent of the total time to boot the server system. The hard drive consumes a significant share of the power used by a computer system, contributes substantially to thermal load, and causes the PC to be noisy due to the fan noise needed to cool the system.
Methods have been developed for increasing the speed in booting the computer system and restoring the computer system from hibernation. One example is disclosed in to Feigenbaum et al U.S. Pat. No. 5,307,497. The '497 patent teaches that a portion of an operating system can be stored in read-only memory (ROM) for fast access of the portion during power-on or reset. However, the portion stored in ROM can not be changed. In other words, the method disclosed in the '497 patent does not adapt to changing user preferences regarding operating system settings and applications used, to restoring the computer system from hibernation, or to updated or new versions of an operating system.
Another example developed by industry is storing the state of the computer memory to a backup memory that is separate from the disk drive and stores the entire computer memory. This backup memory is large because it stores the entire computer memory.